For now, we preserved the "Tor is not working in this browser" mode: when the Tor check fails, the about:tor page displays the following text on a red background (#A4000F) :

Something Went Wrong!
Tor is not working in this browser.

We used the CSS and HTML method to render the onion pattern that is included at the bottom of the page. We included 14 onions across in each row, which means if someone has a really wide display the shapes will not fill the entire width of their window. I think this is OK.

We cannot use the @2x and @3x images because the display density is spoofed and always appears to the CSS rules as 1.0 when privacy.resistFingerprinting is true. We will need to think about how to fix that problem.

One comment on the design: we should consider reducing the empty vertical space above and below the search box. As it is, on many laptop displays the onion design is mostly off the bottom of the page, and on smaller displays (e.g., a 14" Win10 laptop) the link to the manual and the Tor mission text is not visible without scrolling.

Looks good! One additional thing I am wondering is whether we should make our "Questions? Check our Tor Browser Manual »" part larger/show it more prominently. After all, it provides useful information to actually stay secure with Tor Browser and is not just about questions that are showing up during usage.